dm thin metadata: try to avoid ever aborting transactions

Committing a transaction can consume some metadata of it's own, we now
reserve a small amount of metadata to cover this.  Free metadata
reported by the kernel will not include this reserve.

If any of the reserve has been used after a commit we enter a new
internal state PM_OUT_OF_METADATA_SPACE.  This is reported as
PM_READ_ONLY, so no userland changes are needed.  If the metadata
device is resized the pool will move back to PM_WRITE.

These changes mean we never need to abort and rollback a transaction due
to running out of metadata space.  This is particularly important
because there have been a handful of reports of data corruption against
DM thin-provisioning that can all be attributed to the thin-pool having
ran out of metadata space.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

2 files changed, 100 insertions, 9 deletions

diff --git a/drivers/md/dm-thin-metadata.c b/drivers/md/dm-thin-metadata.c
index 72142021b5c9..74f6770c70b1 100644
--- a/drivers/md/dm-thin-metadata.c
+++ b/drivers/md/dm-thin-metadata.c
@@ -189,6 +189,12 @@ struct dm_pool_metadata {
         sector_t data_block_size;
 
         /*
+         * We reserve a section of the metadata for commit overhead.
+         * All reported space does *not* include this.
+         */
+        dm_block_t metadata_reserve;
+
+        /*
          * Set if a transaction has to be aborted but the attempt to roll back
          * to the previous (good) transaction failed.  The only pool metadata
          * operation possible in this state is the closing of the device.
@@ -816,6 +822,22 @@ static int __commit_transaction(struct dm_pool_metadata *pmd)
         return dm_tm_commit(pmd->tm, sblock);
 }
 
+static void __set_metadata_reserve(struct dm_pool_metadata *pmd)
+{
+        int r;
+        dm_block_t total;
+        dm_block_t max_blocks = 4096; /* 16M */
+
+        r = dm_sm_get_nr_blocks(pmd->metadata_sm, &total);
+        if (r) {
+                DMERR("could not get size of metadata device");
+                pmd->metadata_reserve = max_blocks;
+        } else {
+                sector_div(total, 10);
+                pmd->metadata_reserve = min(max_blocks, total);
+        }
+}
+
 struct dm_pool_metadata *dm_pool_metadata_open(struct block_device *bdev,
                                                sector_t data_block_size,
                                                bool format_device)
@@ -849,6 +871,8 @@ struct dm_pool_metadata *dm_pool_metadata_open(struct block_device *bdev,
                 return ERR_PTR(r);
         }
 
+        __set_metadata_reserve(pmd);
+
         return pmd;
 }
 
@@ -1820,6 +1844,13 @@ int dm_pool_get_free_metadata_block_count(struct dm_pool_metadata *pmd,
         down_read(&pmd->root_lock);
         if (!pmd->fail_io)
                 r = dm_sm_get_nr_free(pmd->metadata_sm, result);
+
+        if (!r) {
+                if (*result < pmd->metadata_reserve)
+                        *result = 0;
+                else
+                        *result -= pmd->metadata_reserve;
+        }
         up_read(&pmd->root_lock);
 
         return r;
@@ -1932,8 +1963,11 @@ int dm_pool_resize_metadata_dev(struct dm_pool_metadata *pmd, dm_block_t new_cou
         int r = -EINVAL;
 
         down_write(&pmd->root_lock);
-        if (!pmd->fail_io)
+        if (!pmd->fail_io) {
                 r = __resize_space_map(pmd->metadata_sm, new_count);
+                if (!r)
+                        __set_metadata_reserve(pmd);
+        }
         up_write(&pmd->root_lock);
 
         return r;
diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index 7bd60a150f8f..aaf1ad481ee8 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -200,7 +200,13 @@ struct dm_thin_new_mapping;
 enum pool_mode {
         PM_WRITE,               /* metadata may be changed */
         PM_OUT_OF_DATA_SPACE,   /* metadata may be changed, though data may not be allocated */
+
+        /*
+         * Like READ_ONLY, except may switch back to WRITE on metadata resize. Reported as READ_ONLY.
+         */
+        PM_OUT_OF_METADATA_SPACE,
         PM_READ_ONLY,           /* metadata may not be changed */
+
         PM_FAIL,                /* all I/O fails */
 };
 
@@ -1371,7 +1377,35 @@ static void set_pool_mode(struct pool *pool, enum pool_mode new_mode);
 
 static void requeue_bios(struct pool *pool);
 
-static void check_for_space(struct pool *pool)
+static bool is_read_only_pool_mode(enum pool_mode mode)
+{
+        return (mode == PM_OUT_OF_METADATA_SPACE || mode == PM_READ_ONLY);
+}
+
+static bool is_read_only(struct pool *pool)
+{
+        return is_read_only_pool_mode(get_pool_mode(pool));
+}
+
+static void check_for_metadata_space(struct pool *pool)
+{
+        int r;
+        const char *ooms_reason = NULL;
+        dm_block_t nr_free;
+
+        r = dm_pool_get_free_metadata_block_count(pool->pmd, &nr_free);
+        if (r)
+                ooms_reason = "Could not get free metadata blocks";
+        else if (!nr_free)
+                ooms_reason = "No free metadata blocks";
+
+        if (ooms_reason && !is_read_only(pool)) {
+                DMERR("%s", ooms_reason);
+                set_pool_mode(pool, PM_OUT_OF_METADATA_SPACE);
+        }
+}
+
+static void check_for_data_space(struct pool *pool)
 {
         int r;
         dm_block_t nr_free;
@@ -1397,14 +1431,16 @@ static int commit(struct pool *pool)
 {
         int r;
 
-        if (get_pool_mode(pool) >= PM_READ_ONLY)
+        if (get_pool_mode(pool) >= PM_OUT_OF_METADATA_SPACE)
                 return -EINVAL;
 
         r = dm_pool_commit_metadata(pool->pmd);
         if (r)
                 metadata_operation_failed(pool, "dm_pool_commit_metadata", r);
-        else
-                check_for_space(pool);
+        else {
+                check_for_metadata_space(pool);
+                check_for_data_space(pool);
+        }
 
         return r;
 }
@@ -1470,6 +1506,19 @@ static int alloc_data_block(struct thin_c *tc, dm_block_t *result)
                 return r;
         }
 
+        r = dm_pool_get_free_metadata_block_count(pool->pmd, &free_blocks);
+        if (r) {
+                metadata_operation_failed(pool, "dm_pool_get_free_metadata_block_count", r);
+                return r;
+        }
+
+        if (!free_blocks) {
+                /* Let's commit before we use up the metadata reserve. */
+                r = commit(pool);
+                if (r)
+                        return r;
+        }
+
         return 0;
 }
 
@@ -1501,6 +1550,7 @@ static blk_status_t should_error_unserviceable_bio(struct pool *pool)
         case PM_OUT_OF_DATA_SPACE:
                 return pool->pf.error_if_no_space ? BLK_STS_NOSPC : 0;
 
+        case PM_OUT_OF_METADATA_SPACE:
         case PM_READ_ONLY:
         case PM_FAIL:
                 return BLK_STS_IOERR;
@@ -2464,8 +2514,9 @@ static void set_pool_mode(struct pool *pool, enum pool_mode new_mode)
                 error_retry_list(pool);
                 break;
 
+        case PM_OUT_OF_METADATA_SPACE:
         case PM_READ_ONLY:
-                if (old_mode != new_mode)
+                if (!is_read_only_pool_mode(old_mode))
                         notify_of_pool_mode_change(pool, "read-only");
                 dm_pool_metadata_read_only(pool->pmd);
                 pool->process_bio = process_bio_read_only;
@@ -3403,6 +3454,10 @@ static int maybe_resize_metadata_dev(struct dm_target *ti, bool *need_commit)
                 DMINFO("%s: growing the metadata device from %llu to %llu blocks",
                        dm_device_name(pool->pool_md),
                        sb_metadata_dev_size, metadata_dev_size);
+
+                if (get_pool_mode(pool) == PM_OUT_OF_METADATA_SPACE)
+                        set_pool_mode(pool, PM_WRITE);
+
                 r = dm_pool_resize_metadata_dev(pool->pmd, metadata_dev_size);
                 if (r) {
                         metadata_operation_failed(pool, "dm_pool_resize_metadata_dev", r);
@@ -3707,7 +3762,7 @@ static int pool_message(struct dm_target *ti, unsigned argc, char **argv,
         struct pool_c *pt = ti->private;
         struct pool *pool = pt->pool;
 
-        if (get_pool_mode(pool) >= PM_READ_ONLY) {
+        if (get_pool_mode(pool) >= PM_OUT_OF_METADATA_SPACE) {
                 DMERR("%s: unable to service pool target messages in READ_ONLY or FAIL mode",
                       dm_device_name(pool->pool_md));
                 return -EOPNOTSUPP;
@@ -3781,6 +3836,7 @@ static void pool_status(struct dm_target *ti, status_type_t type,
         dm_block_t nr_blocks_data;
         dm_block_t nr_blocks_metadata;
         dm_block_t held_root;
+        enum pool_mode mode;
         char buf[BDEVNAME_SIZE];
         char buf2[BDEVNAME_SIZE];
         struct pool_c *pt = ti->private;
@@ -3851,9 +3907,10 @@ static void pool_status(struct dm_target *ti, status_type_t type,
                 else
                         DMEMIT("- ");
 
-                if (pool->pf.mode == PM_OUT_OF_DATA_SPACE)
+                mode = get_pool_mode(pool);
+                if (mode == PM_OUT_OF_DATA_SPACE)
                         DMEMIT("out_of_data_space ");
-                else if (pool->pf.mode == PM_READ_ONLY)
+                else if (is_read_only_pool_mode(mode))
                         DMEMIT("ro ");
                 else
                         DMEMIT("rw ");